1. Field of the Invention
This invention relates to linear light-emitting diode (LED) lamps and more particularly to a linear LED lamp with a curved surface to provide a broad viewing angle over 180° along the radial direction.
2. Description of the Related Art
Solid-state lighting from semiconductor light-emitting diodes (LEDs) has received much attention in general lighting applications today. Because of its potential for more energy savings, better environmental protection (more eco-friendly, no mercury used, and no UV and infrared light emission), higher efficiency, smaller size, and much longer lifetime than conventional incandescent bulbs and fluorescent tubes, the LED-based solid-state lighting will be a mainstream for general lighting in the near future. Meanwhile, as LED technologies develop with the drive for energy efficiency and clean technologies worldwide, more families and organizations will adopt LED lighting for their illumination applications. In this trend, the potential safety concerns such as risk of electric shock need to be well addressed.
In many applications of commercial and residential lighting, a linear LED-tube (LLT) lamp is used to replace an existing fluorescent tube, taking advantages of the above said LED's features. In a lighting application of a refrigerated warehouse, an LLT lamp is used to replace a fluorescent lamp because the latter cannot operate at a low temperature of minus 20 degrees Celsius. Use of a high intensity discharge (HID) lamp instead creates much heat and causes the cooling system in the refrigerated warehouse to consume more energy to cool down the refrigerated area. LEDs, however, can operate at minus 40 degrees Celsius, do not generate heat, and thus are well suited for this application. Typical energy savings due to the reduced lighting load are 40%-60% with an additional 12%-19% savings from reduced cooling load.
In high-ceiling lighting applications such as in offices, manufacturing areas, warehouses, showcases in department stores, etc, LLT lamps are used to take advantage of the lowest maintenance cost and the lowest power consumptions and heat dissipations among all kinds of lighting. An LLT lamp can save energy and operating cost by 70%.
A surface mount device (SMD) LED, as a Lambertian emitter, can provide only a beam angle of 120°, in principle. A linear LED tube (LLT) lamp based on surface mount technology inherits this limitation. In some applications such as above mentioned high ceiling areas and refrigerated warehouses, the viewing angle of 180° is required. Some manufacturers, therefore, provide LLT lamps with multiple user-specifiable viewing angles to meet this market demand. They use a variable angle-mounting bracket or rotatable end caps adjusting illumination angle up to 180°. To help install fixtures accurately, they even provide clear bracket featuring angle indicators. Other manufacturers use linear parabolic reflectors and thin-film diffusers to create various beam angles. However, measures such as optics and other means than the present invention can provide only a solution at the expense of extra energy loss due to a limitation of optical efficiency such as transmission, reflection, and absorption loss.
To deal with a wide illumination angle, Timmermans et al. suggests in their patent (U.S. Pat. No. 7,049,761 B2) that a circuit board with an H-shaped cross-section be used. On the horizontal plane of the “H” (horizontal bar in H, extended along the direction to the paper), a plurality of dual-in-line (DIP) LEDs are mounted with different viewing angles against each adjacent one. Because the circuit board that supports LEDs is flat on that plane, the mounting planes for LEDs with different coverage angles must be different to produce an overall predetermined radiation pattern. The DIP LEDs used have a viewing angle between 6° and 45°. For an overall 180° viewing angle, the mounting plane must be between 67.5° and 87° relative to the original plane. One of drawbacks for this design is poor manufacturability, not only in drilling holes at those large oblique angles from the plane normal for mounting DIP LEDs but also in making soldering for each LED connection. Strictly speaking, such drilling at oblique angles between 67.5° and 87° is not manufacturing feasible. Moreover, individual soldering for hundreds of LEDs presents a low-yield, not mentioning inefficiency.
In retrofit application of a linear LED tube (LLT) lamp to replace an existing fluorescent tube, one must remove the starter or ballast because the LLT lamp does not need a high voltage to ionize the gases inside the gas-filled fluorescent tube before sustaining continuous lighting. LLT lamps operating at AC mains, such as 110, 220, and 277VAC, have one construction issue related to product safety and needed to be resolved prior to wide field deployment. This kind of LLT lamps always fails a safety test, which measures through lamp leakage current. Because the line and the neutral of the AC main apply to both opposite ends of the tube when connected, the measurement of current leakage from one end to the other consistently results in a substantial current flow, which may present risk of shock during re-lamping. Due to this potential shock risk to the person who replaces LLT lamps in an existing fluorescent tube fixture, Underwriters Laboratories (UL), use its standard, UL 935, Risk of Shock During Relamping (Through Lamp), to do the current leakage test and to determine if LLT lamps under test meet the consumer safety requirement.
An LLT lamp is at least 2 feet long; it is very difficult for a person to insert the two opposite bi-pins at the two ends of the LLT lamp into the two opposite sockets at two sides of the fixture at the same time. Because protecting consumers from possible electric shock during re-lamping is a high priority for LLT lamp manufacturers, they need to provide a basic protection design strictly meeting the minimum leakage current requirement and to prevent any possible electric shock that users may encounter in actual usage. In other words, when shock hazard happens, the manufacturers have no excuses to claim that they do have proper procedures mentioned in their installation instructions.
Referring to FIG. 1, a conventional LLT lamp 100 comprises a plastic housing 110 with a length much greater than its radius of 30 to 32 mm, two end caps 120 and 130 each with a bi-pin 180 and 190 on two opposite ends of the plastic housing 110, LED arrays 140 and 141 mounted on two PCBs 150 and 151, electrically connected in series using a connector 145, and an LED driver used to generate a proper DC voltage and provide a proper current from the AC main and to supply to the LED arrays 140 and 141 such that the LEDs 170 and 171 on the two PCBs 150 and 151 can emit light. In some conventional LLT lamps, DIP rather than SMD LEDs are used as lighting sources. Although SMD LEDs and the supporting PCB allow more efficient manufacturing, higher yield, higher lumen output and efficacy, and longer life than their DIP counterparts do, some LLT lamp providers still produce such DIP-based products. The two PCBs 150 and 151 are glued on a top plane of the lamp using an adhesive with its normal parallel to the illumination direction. In this case, the viewing angle of the LLT lamp is limited by that of individual LEDs. While SMD LEDs used in the LLT lamp provide a viewing angle less than 120° due to Lambertian emission, a DIP-based LLT lamp offers much less viewing angles.
The bi-pins 180 and 190 on the two end caps 120 and 130 connect electrically to an AC main, either 110 V, 220 V, or 277 VAC through two electrical sockets located lengthways in an existing fluorescent tube fixture. The two sockets in the fixture connect electrically to the line and the neutral wire of the AC main, respectively. The LLT lamp 100 may present electric shock hazard when one of the bi-pins 180 or 190 is first inserted into the socket that connects to the line of AC main. The energized LED driver causes a lamp leakage current flowing through the exposed bi-pin 190 or 180 not in the socket, and thus presents risk of shock during re-lamping.
FIG. 2 is an illustration of another conventional LLT lamp, claiming to have a wider viewing angle. The LLT lamp 1000 comprises a plastic housing 1100 as bulb portion, and an “H” shape circuit board 1200. On the horizontal plane 1300 of “H” is DIP LEDs 1301 mounted. DIP LEDs 1401 and 1501 are mounted on different planes 1400 and 1500, respectively (shown in FIG. 3). No end caps with bi-pin are shown in FIG. 2 for clarity. FIG. 3 is a cross-sectional view of FIG. 2. The LED array 1301 is mounted on the plane 1300 while LED arrays 1401 and 1501 are mounted on the plane 1400 and 1500, respectively, each with their own radiation patterns. In combination, the overall beam has a wider viewing angle in the radial direction than the individual beam does. As mentioned, when the planes 1400 and 1500 incline at large angles to achieve an 180° viewing angle for the overall beam emitted from DIP LEDs, the hole drilling at such oblique angles as 67.5° and 87° relative to the original plane 1300 becomes manufacturing infeasible. As can be seen, the beam angle is far from 180°, partly because the two vertical planes 1600 and 1601 of “H” block part of the beam. DIP rather than SMD LEDs used are another reason that the beam cannot radiate that wide due to the limitation of narrow viewing angle of DIP LEDs.